Some references
Specifically on Brunei
The Seashore Plants. The Seashore Life of the Brunei Heart of Borneo vol. 2. R. McIlroy, C. Obendrauf. 2008. Panaga Natural History Society and Brunei Shell Petroleum, 69 pp.
Beachcombing. The Seashore Life of the Brunei Heart of Borneo vol. 4. R. McIlroy, M. Desvoyes. 2008. Panaga Natural History Society and Brunei Shell Petroleum, 81 pp.
A Field Guide to the Forest Trees of Brunei Darussalam and the NW Borneo hotspot. Volume 1. Ed. by P.S. Ashton, A.S. Kamariah, Idris M. Said. 2003. University of Brunei Darussalam, Brunei Forestry Department, Brunei Shell Petroleum and Sultan Haji Hassanal Bolkiah Foundation. 281 pp.
Forests and Trees of Brunei Darussalam. Ed. by K.M. Wong and A.S. Kamariah. 1999. University of Brunei Darussalam in association with Brunei Forestry Department and Brunei Shell Petroleum, 109 pp.
In Brunei Forests. An Introduction to the Plant Life of Brunei Darussalam. K.M. Wong (with watercolours by C.L. Chan). 1997. Natural History Publications (Borneo) Sdn. Bhd., 127 pp.
The Rattans of Brunei Darussalam. J. Dransfield. 1997. Ministry of Industry and Primary resources, Brunei Darussalam, 217 pp.
A Checklist of the Flowering Plants and Gymnosperms of Brunei Darussalam.
Ed. by M.J.E. Coode, J. Dransfield, L.L. Forman, D.W. Kirkup, Idris M. Said. 1996. Ministry of Industry and Primary resources, Brunei Darussalam, 477 pp.
Common Seashore Life of Brunei. M. Wong and Aziah bte Hj. Ahmad. 1996. Brunei Museum. Ministry of Culture, Youth and Sports, 121 pp.
Treasures of Brunei Darussalam. J. Henrot. 1995. Brunei Shell Petroleum, 26 pp.
Belalong: A Tropical Rainforest. Earl of Cranbrook, D.S. Edwards. 1994. The Royal Geographical Society & Sun Tree Publishing, 389 pp.
The Flora of the Peat Swamp Forests of Sarawak and Brunei, including a catalogue of all recorded species of flowering plants, ferns and fern allies. J.A.R. Anderson. Garden's Bulletin Singapore. 1963. vol. 20: 131-228. (not yet available at the Garden Bull. online).
Healing Flora of the Dusun. http://www.thefreelibrary.com/Healing+flora+of+the+Brunei+Dusun.+%28Research+Notes%29.-a093533247
Plant pictures from Brunei (60), Sarawak (300), Sabah (42), Kalimantan (58) by Martin Dančák. https://picasaweb.google.com/martindancak
Borneo
Field Guide to the Plants of East Sabah. R.P.J. de Kok and T.M.A. Utteridge. 2010. Royal Botanic Garden, Kew, 176 pp.
Gingers of Sarawak. A.D. Poulsen. 2006. Natural Histoty Publications (Borneo) Sdn. Bhd., 102 pp.
Etlingera of Borneo. A.D. Poulsen. 2006. Natural history Publications (Borneo), Sdn. Bhd., 263 pp.
Pitcher Plants of Sarawak. C. Clarke and Ch'ien Lee. 2004. Natural Histoty Publications (Borneo) Sdn. Bhd., 80 pp.
Pitcher-Plants of Borneo. A. Phillipps, A. Lamb and Ch'ien C Lee, 2008. Natural Histoty Publications (Borneo) Sdn. Bhd., 305 pp.
Tree flora of Sabah and Sarawak. Volume 1. Ed. E. Soepadmo and K.M. Wong. 1995. 513 pp.
Tree flora of Sabah and Sarawak. Volume 2. Ed. E. Soepadmo, K.M. Wong and L.G. Saw. 1996. 443 pp.
Tree flora of Sabah and Sarawak. Volume 3. Ed. E. Soepadmo and L.G. Saw. 2000. 511 pp.
Tree flora of Sabah and Sarawak. Volume 4. Ed. E. Soepadmo, L.G. Saw and R.C. Chung. 2002. 388 pp.
Tree flora of Sabah and Sarawak. Volume 5. Ed. E. Soepadmo, L.G. Saw and R.C. Chung. 2004. 528 pp.
Tree flora of Sabah and Sarawak. Volume 6. Ed. E. Soepadmo, L.G. Saw, R.C. Chung and Ruth Kiew. 2007. 335 pp.
Forest Institute Malaysia (FRIM), Sabah Forestry Department, Sarawak Forestry Department.
The tree and shrub genera of Borneo. James K. Jarvie (The Arnold Arboretum of Harvard University) and Ermayanti (Conservation International) http://www.phylodiversity.net/borneo/delta/
South East Asia
Southeast Asian Begonia Database. Hughes, M. & Pullan, M. (2007). Electronic publication accessible via www.rbge.org.uk. http://elmer.rbge.org.uk/begonia/
Ferns of Malaysia in colour. A.G. Piggott. 1988. Tropical Press Sdn.Bhd., Kuala Lumpur, 458 pp.
Garden's Bulletin Singapore online. http://www.sbg.org.sg/research/publicationbulletin.asp?sRange=1960-1969&sYear=1963
Guide to the mangroves of Singapore. From "A Guide to Mangroves of Singapore", P. K. L. Ng and N. Sivasothi (editors) Volume 1: The Ecosystem and Plant Diversity and Volume 2: Animal Diversity BP Guide to Nature Series published by the Singapore Science Centre, sponsored by British Petroleum. 2001 Raffles Museum of Biodiversity Research, The National University of Singapore & The Singapore Science Centre. http://mangrove.nus.edu.sg/guidebooks/text/flora.htm
(A site not related to Brunei or S.E. Asia but worth visiting: